Oxide superconductors, the critical temperature (Tc) of which exceeds liquid nitrogen temperature, have been under extensive studies in expectation of application to wires and devices.
In order for an oxide superconductor to be applied to wires, it is necessary to produce an oxide superconductor having a high critical current density (Jc) and a continuous length. To produce a continuous length oxide superconductor of tape form, an oxide superconductor must be formed on a metal tape from the viewpoint of strength and flexibility.
Because an oxide superconductor is crystallographically anisotropy, it is required to establish a deposition technique in which an oxide superconductor is allowed to grow epitaxially on a textured substrate to achieve improvement of Jc.
MOD process (metal organic deposition process) has been studied as a method for making a tape-shaped Re—Ba—Cu based oxide superconductor film.
MOD process is a method involving pyrolysis of a metal-organic acid salt, in which a uniform solution of a metal-containing organic compound is applied to a substrate and subsequently pyrolyzed to form a thick film on the substrate. The MOD process has advantages of non-vacuum system, high Jc, low cost, and high fabrication speed and is therefore suited to the production of a tape-shaped oxide superconductor.
According to the MOD process, pyrolysis of the starting material (metal-organic acid salt) generally results in the intermediate production of a carbonate of an alkaline earth metal (e.g., Ba). An oxide superconductor is produced through a solid phase reaction of the carbonate. This reaction requires a high temperature treatment at 800° C. or higher. Another significant problem is a reduction in Jc with an increase in film thickness.
To address the above problems, patent documents 1 to 3 listed blow disclose a technique for fabricating an RE-Ba—Cu based oxide superconductor, in which an aliphatic organic acid salt (e.g., an octylate or naphthenate) or a fluorine-containing organic acid salt represented by trifluoroacetic acid is used as a starting material to be calcined and fired.
For example, patent document 3 discloses a composition for thick oxide superconductor film containing a salt of a branched saturated aliphatic carboxylic acid with 6 or more carbon atoms or an alicyclic carboxylic acid with 6 or more carbon atoms as a copper precursor compound; a salt of a branched saturated aliphatic carboxylic acid with 6 or more carbon atoms, an alicyclic carboxylic acid with 6 or more carbon atoms, or a trifluoroacetic acid as an yttrium precursor compound; and barium trifluoroacetate as a barium precursor compound. According to the disclosure, the precursor compounds have good solubility; the composition exhibits excellent coating properties to provide a tape-shaped thick film with uniform thickness and electrical characteristics at a high speed; and a uniform tape-shaped oxide superconductor is obtained at a rate of temperature rise of about 5° C./min in calcination.
In order to establish industrial scale production of continuous length tape, further improvement of production efficiency is needed and, from this viewpoint, it has been demanded to further increase the rate of temperature rise in calcination. If the rate of temperature rise in calcining the composition for a thick oxide superconductor film of patent document 3 is increased, however, the tape would have a larger thickness along edges than in the laterally middle portion, which can result in problems such as non-uniformity of electrical characteristics and crack development.
Patent document 4 discloses a salt of a keto acid, such as acetoacetic acid, propionylbutyric acid, or benzoylformic acid, as an MOD starting material (metal organic acid salt) for an oxide superconductor.